Synthetic resins such as polyolefin have problems in that, for example, when used by themselves, they are rapidly degraded by heat and ultraviolet light during processing and use and thus do not withstand use. Therefore, such synthetic resins are blended with a variety of additives such as an antioxidant, a UV absorber, a hindered amine compound, a nucleating agent, a flame retardant and an antistatic agent.
Conventionally, these additives are added in the form of powder having excellent dispersibility in a resin; however, with consideration for product contamination caused by dust and worker's health, granulation and masterbatching of such additives have been demanded. For example, there have been proposed granulation methods in Patent Documents 1 and 2 and masterbatching methods in Patent Documents 3 and 4.
Masterbatching is an excellent anti-dust measure; however, the thermal history of the resin used becomes extended and the physical properties of the resulting final product are likely to be affected depending on the resin used. In particular, since an additive having poor compatibility with a resin cannot be masterhatched at a high concentration, there is a problem that the ratio of the resin used for the masterbatching contained in a final product becomes high, so that the design of the product composition is constrained. On the other hand, granulation products contain no or only a small amount of a resin component; therefore, they are superior in that arbitrary composition and physical properties of a final product can be easily set.
There are also granulation methods which utilize a binder; however, since a binder affects the resin physical properties, granulation is preferably performed with the required additive(s) alone. Meanwhile, phenolic antioxidants having a melting point of not higher than 200° C. are widely used because, when they arc melted and allowed to function as a binder, other high-melting-point additives can be granulated.